1. Field of the Invention
The invention relates to a nitride semiconductor free-standing substrate, and a method for making the nitride semiconductor free-standing substrate. Particularly, the invention relates to the nitride semiconductor free-standing substrate wherein a warpage of the free-standing substrate is reduced, and a method for making the nitride semiconductor free-standing substrate.
2. Description of the Related Art
A conventional method for making a GaN single crystal substrate as a nitride semiconductor free-standing substrate is conducted such that a nitride semiconductor layer is hetero-epitaxially grown by using a vapor-phase growth method on a heterosubstrate, which is different from the nitride semiconductor, such as a sapphire substrate, a silicon substrate, and a gallium arsenide substrate, and the heterosubstrate is then removed by polishing, etching or peeling to leave only the nitride semiconductor layer, whereby a nitride semiconductor free-standing substrate is formed (e.g., see JP-B-3788041). This method involves a step that a mask having dotted windows aligned in [11-2] direction at constant intervals and shifted a half pitch in [−110] direction is formed on a (111) GaAs substrate, a GaN buffer layer is grown on the mask formed, GaN crystal is epitaxially grown on the GaN buffer layer, and the GaAs substrate and the mask are then removed to have the GaN free-standing substrate.
However, in the method described in JP-B-3788041, many defects arise at the initial stage of the growth of the nitride semiconductor crystal since the nitride semiconductor crystal is grown on the heterosubstrate mismatching to the nitride semiconductor. Thus, when the heterosubstrate is separated from the nitride semiconductor crystal, there occurs a difference between the defect density at the surface of the nitride semiconductor crystal and the defect density at the bottom thereof. Due to the difference in defect density, internal stress is left in the nitride semiconductor crystal. Consequently, a warpage arises in the nitride semiconductor free-standing substrate after separating the heterosubstrate.
In the nitride semiconductor free-standing substrate having such warpage, plane orientations are not orientated in a uniform direction in a plane thereof. Therefore, dispersion must be generated among in-plane off-angles even when the nitride semiconductor free-standing substrate is flattened by polishing or the like. In producing a light-emitting device by using the nitride semiconductor free-standing substrate having the dispersion among in-plane off-angles, dispersion will be generated in its emission wavelength due to the dispersion among in-plane off-angles. Along with this, a further problem will arise that the production yield is reduced with respect to the light-emitting device to emit light of a desired wavelength.